Various types of liquids, such as glue or adhesive, or other viscous liquids, are dispensed from a syringe using a timed pressure pulse. The dispensing operation result in the formation of a dot of liquid on a substrate, such as an electronic circuit board. In many applications the size or volume of the dot dispensed during the timed pressure pulse is small and must be held to strict tolerances.
Known syringe dispensers often utilize pressurized actuation air to force the liquid from the syringe. That is, a short burst of pressurized air into the syringe forces the small amount of liquid from the syringe. After successive pulsed dispensing operations, the volume of the liquid in the syringe decreases, and the remaining volume in the syringe must fill with pressurized actuation air. As the air volume or space in the syringe increases due to the concurrent decrease in space occupied by the liquid volume, the time to reach the actuation air pressure increases. Therefore, for short dispense cycle times the proper actuation air pressure will not be reached, and this will affect the dot size or liquid output volume of the successive dispense cycles. More specifically, if the dispensing system does not compensate for this situation, the liquid discharge volume or dot size will decrease as the volume of the liquid retained in the syringe decreases. One known compensation method is to use an actuation air pulse of varying time to compensate for the changing air and liquid volume in the syringe. That is, as the liquid volume in the syringe decreases, the duration time of the actuation air burst is increased.